Cracking Open the IBM PC Jr.

The junior PC that could

The PC Jr. was IBM's attempt to tap into the budding home personal computer market of 1983. While the IBM PC Jr. had many positives with regard to the general home user, it also had several limitations that doomed it in the marketplace. I bought my PC Jr. in 1985 from my older brother who never quite figured out what to do with it. Feeling nostalgic, I decided to Crack Open the IBM PC Jr. to see what was actually in the case.

No screws

One of the first things you notice about the PC Jr. when you go to crack it open is that there are no screws on the outside of the case.

I wonder why all the manufacturers decided to go with cases shut tight with screws in the 20 years or so after this PC was on the market.

Bona fide

The Serial Number and pertinent FCC Regulation information for our particular IBM PC Jr. We don't have a modem in ours so the second regulation does not apply.

Side panel expansion

One of the interesting and unique features of the PC Jr. was the side panel expansion system.

Here we are looking at the expansion panel for a parallel printer port. My personal PC Jr., all those years ago, had a 128KB RAM expansion board. (That's right, a whopping 256KB of RAM total)

Door of expansion

The plastic door of the expansion pops right off to reveal several screws and another port of an additional expansion if you so wished.

Double set

These screws are a bit of a wild goose hunt. They only apply to the expansion card and they are double set screws. By that I mean, the first part of the holds the expansion card to the case and the second part holds the expansion card together. But we'll get to that later.

Opening the main case

The top of the main case is held in place by tabs. These tabs (red arrows) are released with a gentle twist of a flat-head screwdriver.

Our first look inside

The layout is very utilitarian - making judicious use of expansion slots.

It is also very dusty - our poor PC Jr. has obviously been neglected. But it still works - they don't make'em like they used to I suppose.

The floppy drive

The 5 1/4 floppy drive in the IBM PC Jr. is remarkable for its size -- it is huge and takes up one whole side of the case.

Expansion slots

The other side of the case is more open and includes an open expansion slot for an internal modem.

Expansion slot layout

There are four pieces of equipment highlighted (red arrows):

The floppy disk controller

The infrared reader for the keyboard

The RAM expansion (64KB)

The power supply controller board

Expansion cards removed

The expansion cards are removed easily and the motherboard begins to be revealed.

Empty expansion slots

The red arrows mark where the floppy controller, RAM expansion, and power controller were connected to the motherboard. The IR sensor is still installed, but will be removed before long.

Floppy disk removed, motherboard revealed

Oddly enough, that huge floppy drive is not bolted to the motherboard and it lifts right out revealing the entire motherboard underneath.

The red arrows mark the cartridge slots

The yellow highlight is the CPU

The blue arrows mark where the expansion modules connected

The green highlight marks the primary RAM memory bank

The floppy controller

The floppy controller matches the floppy drive itself in terms of huge size. All of this fits onto a small chip these days.

Jump the connection

This is interesting - for some reason IBM used an external wire soldered between contact points. There are actually two here.

Jump the connection revisited

A connection is also jumped with a soldered external wire on the back side of the board.

Multinational

The floppy controller has chips from all over the world. Some of the manufacturers represented in this chip layout include Texas Instruments, Motorola, and National Semiconductor.

RAM expansion - shielding

The RAM expansion card is shielded with a metal casing.

RAM expansion revealed

After removing the shielding we can see a fairly standard set of RAM chips and a few controllers.

Power control

The power controller is compactly designed but looks ominous and should be handled with care.

A very large capacitor

The black large capacitor near the on / off switch is very prominent. It is also very dusty.

Coils

From the opposite angle you can see a couple of wire coils, one of which is inside a metal shield.

Connection jumping

The power supply controller board also has a externally jumped connection.

Floppy drive

The 5 1/4 floppy drive is rather large even though the floppy disks could only hold 360KB. Besides the rather large number of chips there are also mechanical components and motors.

Qume

The floppy drive maker is Qume, which is not familiar to me.

Usual suspects

The chips on the floppy drive mirror the chips we saw on the controller from Texas Instruments, Motorola, and National Semiconductor.

Infrared sensor

The infrared sensor for the wireless keyboard is not very big, breaking the trend set by the floppy drive.

IR sensor eye

The red arrow marks the eye of the infrared sensor.

CPU

The IBM PC Jr. uses the Intel P8088 (red arrow) CPU, which was a very common CPU at the time. The yellow arrow marks a Programmable Interrupt Controller.

Advanced Micro Devices

Before they went into the CPU cloning business, AMD made chips that supplemented the activity of the Intel 8088. The P8253-5 is a Programmable Interval Timer and the P255A is a Programmable Peripheral Interface.

Linear Encoder

This large chip is a Linear Encoder with IBM's name on it.

RAM chips

The red arrows mark the bus chips from our triumvirate of chip manufacturers, Motorola, National Semiconductor, and Texas Instruments.

The yellow box indicates one bank of the RAM chips from Mostek. Each chip is 8KB, making for a total of 64KB when you account for all eight chips.

Video chip

The IBM PC Jr. does not have what we would consider a dedicated GPU. Instead it has the Motorola CRT controller marked in yellow.

Marked with a red arrow below that is a technical name Asynchronous Communication Interface Adapter. To me, that means modem - this chip would help process incoming information from an external modem.

Just to the right of the CRT controller is a chip, upside down in this photo, with the number SN76496N. That is the Texas Instrument sound chip.

Mystery chip

The chip marked in yellow may be important, but I found it curious. I also found that I could not find information explaining what its function was. I would guess it has something to do with either the RAM expansion or the Internal modem.

Perhaps someone out there can share some insight concerning the chip in question?

More memory

The yellow box shows all of the primary RAM chips from Mostek. The red arrows show where the infrared module connects to the mother board.

Showing age

The red arrows point to the expansion slots and expose the fact that our PC Jr. is old. The connections, once bright and shiny, show definite signs of oxidation.

ROM

The other prominent chips on the motherboard are the three ROM chips marked in yellow.

Back to BASIC

These two IBM chips marked in yellow provide the PC Jr. with BASIC. BASIC is sort of the default operating system in the PC Jr.

Mux/Dux

Set the time please

Parallel

The circuit board in the parallel port expansion module is fairly standard.

Bend it like Beckman

Although there is a strange white chop on board. Anyone care to take a guess on that?

Cracking Open

Here is the IBM PC Jr. all cracked open.

The PC Jr. holds a special place in my heart as my first IBM-based PC. Although I quickly out grew it, this version of the IBM personal computer served me well that last semester of business school where I had to produce several term papers. There is nothing like being able to pull an all-nighter from the comfort of you own living room rather than the school computer lab.

Mux/Dux

About Mark Kaelin

Mark Kaelin is a CBS Interactive Senior Editor for TechRepublic. He is the host for the Microsoft Windows and Office blog, the Google in the Enterprise blog, the Five Apps blog and the Big Data Analytics blog.

Full Bio

Mark Kaelin is a CBS Interactive Senior Editor for TechRepublic. He is the host for the Microsoft Windows and Office blog, the Google in the Enterprise blog, the Five Apps blog and the Big Data Analytics blog.

While the copyright you show on the PCjr is 1983, the PCjr was announced in February, 1987. I know because I did the announcement for my IBM branch office and I was 7 months pregnant with my son, who was born in April 1987. At the announcement I told the audience we were "adding to the PC family" and "I wasn't sure why they picked me to do the announcement." I got a good laugh. The copyright was likely acquired while the machine was in early development.

I remember this little puppy and some of those other great failures from IBM for example PS2s with micro channel architecture and WARP..... What a ya do when WARP fails go to impulse power. They thought they were so big they could go it alone and change the market place... hahahaha

The person writing the comments to the pictures of this breakdown doesn't seem to have a clue about basic computer technologies almost 30 years back. Those solder "fixes" were quite common those days to fix initial design flaws or to make simple repairs.
And a 8250 is not directly used for a modem but it is a gernal UART/serial port chip.
And it looks like he hasn't seen some real dust in an old computer either...

I still have several of these stored away - one even has a Quadram expansion chassis (looks like another jr stacked on top).
What I'd really like to have is one of the Legacy expansion units with all the flashing LEDs :)

I worked for the company that published "Jr." magazine, devoted to the PC Jr. When our tech editor returned from the first pre-release demo of the computer, the first two words out of his mouth were "Chiclet keyboard." What a colossal disappointment!
Of all the ways to avoid competing with the IBM PC, the toy keyboard was possibly the worst. At a time when a new computer still needed the support of hardcore techies to be taken seriously, the Chiclet keyboard guaranteed the Jr. would be ridiculed, even with Chaplin as its spokesperson.
Jeff DeTray
http://www.AstronomyBoy.com

I used to do a lot of dBase programming on it. Not the best machine but it replaced my Osborne which had become sadly useless due to MS-DOS replacing CPM. I still have my original ST506 and ST412 disks from that era. Hard to think now we have 3TB units at a fraction of the size.

The 8250 uart was used as serial ports, either for modems or serial printers.
The beckman is a resistor network 4.7Kohm, now they are still used but measure only approx 1/8th of an inch. As for AMD they were making slice processors for mini computers much more powerfull than the 8088.

The 8088 Intel (or AMD) CPU could be extracted, and replaced with a NEC V20 chip, whose speed was a whopping 9 Mhz. Twice as fast as the Intel/AMD. I worked on quite a few of these vintage machines. Some had Intel CPUs in them, some had AMDs. Whatever was in it, it was an 8088 chip. So you see, AMD was a secondary vendor for CPUs for IBM I guess.
Juinor was the PC that got me hooked. With jr, and a modem, I could get on all the local BBSes, and swap shareware with friends. I learned BASIC on the thing, not that I am a programmer.
It kept me company many evenings, while I learned what makes PCs tick. Hard to believe it was almost 30 years ago... Perhaps it's time I retire from this.

I remember Qume when I worked in the Civil Service (UK). There was a lot of really old kit there. I think they made printers.
As for the Beckman (white) chip, I think it's a resistor network. Maybe 8 resistors each 4.7k ohms.

Someone mentioned Vacuum Tubes, or what we Brits called (Thermionic) Valves .Ah the love of my life! I used to work with these in my teens in 1967 when I took up electronics as a hobby.
The nomenclature was different in the USA and the UK, for example what would have been a 6V6G in the USA would have been a EL34 in the UK. It is an (Audio) Output Pentode.
In the UK (Mullard) system the first letter was the heater voltage, the second the valve type, the first number the base type and the second number if it was straight = even or variable Mu = odd. Thus the EL34 had a 6.3 volt heater, was an Output Pentode, with an International Octal Base and was straight. As an another example a UABC80 would have been used in an FM Radio with a series heater chain , and was a triple diode triode with a nine pin B9A base. My eyes still go misty when I think of value (tube) circuitry but I simply can't find the same pleasure from modern solid-state electronics.

Can't comment specifically on the PC JR, but jumpers were used on the pre-PC, mini's (desk size 8K computers) of the 70's to enable additional features. The hardware for the lowest to the highest model was the same, but the jumpers enabled various features as the price went up.

Still have a "luggable" version of this machine. Get it out and fire it up once in a while just for grins. (remember booting off of a DOS single sided floppy anyone) The technology certainly has gone a long way..

It is fun to read all the comments. My husband worked for IBM for 30+ years and we got our PC jr for our son to use in college. We gave it away to a young techie a few years ago who was thrilled to get it. He may have taken it apart as you did, or experimented with using it. It still worked!

This was my first computer back in the day. My father worked for IBM for 35 years in upstate NY. He was able to purchase one as an employee for a discount. Even with that the PC, monitor and printer still cost upwards of $3K. We had 2 memory expansion cards on the side for a huge 512K RAM. Sure made playing King's Quest fun. Hard to beleve this was almost 30 years ago.

I Sold a few in 83-84, IBM just started to promote them along with the IBM AT That came out at the same time. The PC Jr's wireless IR keyboard was a way ahead of it time. But they did't sell, just to little to run on it and the cost was hight. Apple was out with the Macinosh 32K around that same time. It did better. and then there was the Apple IIc - yeck! more on that later... Yes the good old days

... two of the extra connections you mention are, in fact, resistors that have been omitted from PCB layout (remember, CAD software was kinda rare these days, so routing was most likely done by hand), and
... AMD was in the cloning business even then - those 8253 and 8255 chips were actually designed by Intel (which is why the labels begin with "8", just like 8088)

The Beckman chip is a bunch of resisters in a DIP-housing.
The 89 is the type of the chip.
The next 8 means a 16 pins DIP-housing. (the 9 was 14 pins).
Then follows the resistor configuration. The 1 means that the resistor are in a bus-configuration with the common to pin 16.
It seams to be a chip with 4.7K resistors in it.
So, just an easy way to install resistors on a printed-circuit-board.
ThQ.

Here in New Zealand we got a Japanese version of the Jr called the JX. It used the same design concept but the hardware was quite different, we had a metal case and the first (if think) 3.5 inch disks in it. It could also take an expansion box that included a hard disk. There was also a full size keyboard available. Main weakness was the NEC colour screen, mine blew a power transistor that desoldered itself from the circuit board. KQ1 would only play on the Jr and JX, no other first generation PX could run it because of screen colour limitations.

What a great little machine, so way ahead in many ways, color, great sound, tiny chiclet wireless keyboard, and a fantastic platform for games, easy to take it in the car on trips away. I can remember paying $50 for 10 diskettes, and and exhorbitant amounts to add a printer port (nice the way the modules bolted on the side). Great times having a crowd of friends playing Kings Quest. Still got mine in its box stored away.
Phil H

I used a modified version (640 KB memory) on my sailboat for several years for getting weather faxes over SSB as well as sending text over Ham radio with a PK-232.
http://www.timewave.com/support/PK-232/PK232DSP.html
Also doubled as a word processor etc. Never failed once even though it was in 100% humidity all the time. Great little machine - brings back a lot of memories!

I worked as an engineering tech at the Teledyne plant in Lewisburg Tennessee where the PC Jr. was built. The PC Jr. was built specifically for the then unknown home market. It was felt that the home market would not want a steal case. That is why the case was plastic. I think that the plastic case actually cost more than a metal one would have. To keep RF emissions down to pass FCC rules, the inside of the plastic case was sprayed with some sort of nickel coating. Even at that time nickel was not cheap! Of course a steal case would not have emission problems. Next let?s get the time line correct. The IBM XT had just become available shortly before the PC Jr. started shipping. The motherboard was far superior to that of the XT. For instance, for the XT to be able to have a color monitor, it required a full size expansion board (from the front of the case all the way to the back of the case). The PC Jr. did this with 1 gate-aray chip on the mother board. This expansion board also cost more than the entire PC Jr. The PC Jr. had less memory because the plan was to use ROM cartridges for all application. These were the slots located under the disk drives. The floppy controller was capable of running 2 floppy drives, but I don?t think the external drive ever became available to the public.
On the down side the keyboard was bad and the power supply was inadequate. The power supply only had enough power to run the memory expansion card and the parallel printer card. There was an optional second power supply that connected to the explanation slot on the side of the case if more options were added. There was also an option for a disk drive controller that was capable of running 2 hard disk drives. I don?t think either option sold very well.
If IBM had gone after the business market first, I think the PC Jr. would have been more advanced than the XT! They just could not justify increasing the price of the PC Jr. to get the same revenue enjoyed by the XT after pricing for the home market first.
Thanks for the trip down memory lane and my first job out of school! The pictures were great!

Qume was an early PC drive manufacturer who was bought many years ago by I forget who (maybe Maxtor?). They did make quality, high capacity (for the time) hard drives as I recall.
The wires on those boards usually indicate an engineering correction change. If an engineer puts out a design that requires too many of these and he's back to "flipping burgers." As I remember, the PCjr had many teething problems with its hardware requiring many engineering changes -- another factor contributing to its demise.

Notice that the Belfuse 8333 is mounted in between plug in sockets. Undoubtedly it is a power supply for the sockets, but it was probably meant to act as a fuse box to keep a bad board from destroying the mother board. You could also use low wattage resistors that would do the same thing. They would control the voltage and burn out if the current flow got to great. From a tube TV repair technician. Of course they developed very low current resistor-fuses. Belfuse is still making fuses. Take a look at www.belfuse.com

Near as I recall, the PC Jr. was not released in 1987, it was discontinued that year.
My memory could be fading but as far as I recall, the company I worked for in 1984-86 purchased one and then promptly dumped it as being unusable. I left there in early 1986 but I still remember a couple of friends who still worked there getting an Xmas present in the shape of finding the doors locked and the offices cleaned out when they came to work on Friday, Dec. 19th. The staff's personal stuff was either taken or thrown in the dumpster.

I poke around tube type amplifiers and short wave radios for fun. Nothing beats the even-order distortion you get from a tube. Solid state is fundamentally unnatural, no matter how you try to process the signal it has agitation built in, and degradation of the signal piled on in order to mitigate the harshness.
Give me tubes for audio any day. BTW 6V6 is my favorite for guitar amplification. The greatest tube type amp ever built in my esteem is the Ampeg SVT, six 6550 output tubes, 14 tubes overall. With one of those monsters you don't need to "go to eleven," "one" is about all you can take.
You know what else is cool; a tube-type expander/limiter. The 60's wouldn't have been the 60's without it. Pop music on AM radio would have sounded flat.
Plus you don't get the beautiful glow of Rayleigh refraction in a silicon chip! Nothing beats seeing a cloud of electrons waiting to rush out through the anode into a 20 pound iron output transformer!

I did the same thing in the early 90's
The SSB speaker output went to a rs232 port adapter and was used to decode military weather faxes on the US east coast.
Computer was used to reduce celestial readings, store tide tables, log, and storage lists.
Boat was home for 20 years and cruised from 90 to 95.
http://dotcom-productions.com/images/reg/seadove/sea-dove.htm
54' Sampson C-Breeze ferro cement full keeled ketch.
Reg

I had 640k of memory installed on my PC Jr. as well, and used compiled BASIC and Assembler to write a program to do zoomed fractals. The first deep zoom took 4 hours less than 3 weeks of 24x7 runtime to produce a picture. I still have a photo of it.

Exactly right, except "8333" is probably the year and week of manufacture. The parts name is (was) 0447-0150-90...
That was the time of the TTL-graveyards with 5 Volt lines as thick as welding cables and MathCo's at the price of a used car...